The invention disclosed in this specification relates to a fabrication method for a thin-film semiconductor utilized in devices using thin-film semiconductors (for example, thin-film transistors, photo-electric conversion devices, etc.).
In recent years liquid crystal display devices utilizing thin-film transistors have become well-known. These are known as active matrix type devices, and have thin-film transistors respectively arranged in each pixel disposed in a matrix shape, these thin-film transistors controlling input and output of charges maintained in the pixel electrode of each pixel. These types of active matrix liquid crystal display devices are compact and light-weight, and in addition since they can display a high speed picture in minute detail, are expected to become the main force in future display devices.
Thin-film transistors which are utilized in active-matrix liquid crystal display devices require to be formed on the surface of a substrate having translucence. This is because light is required to pass through the substrate forming the liquid crystal display.
As a substrate having translucence, a glass or quartz substrate, or even a plastic substrate may be cited. In forming a thin-film semiconductor, since heating must be performed to a certain extent, utilizing a plastic substrate is inappropriate. Also, since a quartz substrate can withstand high temperatures in the order of 1000° C., it is appropriate as a substrate for forming a thin-film semiconductor, although it is generally unsuitable due to its high cost (in particular, over a large area it can be ten times the cost of a glass substrate or more).
Consequently, a glass substrate is generally used, the thin-film semiconductor being formed on the surface of this glass substrate. Currently, as a thin-film semiconductor, an amorphous silicon film is generally used. The amorphous silicon film can be formed by a plasma CVD method and heated to about 200 to 400° C., therefore a low-cost glass substrate can be utilized.
Also, where fabricating a thin-film transistor using an amorphous silicon film, there is the problem that the characteristics thereof are low. Accordingly, in order to achieve an active matrix liquid crystal display device having a display characteristic which is more effective than that obtainable under current circumstances, a thin-film transistor having an even higher characteristic is necessary.
In attaining a thin-film transistor having an even higher characteristic than a thin-film transistor using an amorphous silicon film a crystalline silicon film may be used as the thin-film semiconductor. A crystalline silicon film can be achieved by thermal processing of an amorphous silicon film. However, in such a case the following problems occur. Namely, although generally the withstand temperature of a glass substrate is 600° C. or less, crystallization of an amorphous silicon film requires temperatures of 600° C. and more. Thus techniques of performing thermal processing at a temperature of around 600° C. to crystallize an amorphous silicon film formed on a glass substrate are currently being researched. However, where crystallizing an amorphous silicon film at a temperature of about 600° C., it is necessary to perform thermal processing for some tens of hours or more (generally 24 hours or more), therefore there is the problem that practicality and productivity are extremely low.
As a technique for solving this problem, there is a technique of deforming the amorphous silicon film into a crystalline silicon film by irradiating it with a laser beam. Since irradiation by laser beam does not incur thermal damage to the lower level (base) glass substrate, the problem of thermal resistance of the glass substrate accompanying a method using thermal processing does not occur.
However, where an amorphous silicon film of about 1000 Å or less is irradiated by a laser beam, it is clear that corrugations form in the surface of the crystalline silicon film thus obtained. This tendency is particularly strong where the amorphous silicon film, which is the starting film, is thin at 1000 Å or less. Alternatively, from the problem of laser beam absorption the result that the thinner the film thickness (particularly 500 Å or less) of the amorphous silicon film which is the starting film the more favorable for crystallization.
Namely, where the thickness of the amorphous silicon film which is the starting film is made thin in order to facilitate crystallization, there exists the dilemma that the surface of the thus-obtained crystalline silicon film will have large corrugations.
FIG. 2 shows the state of the surface of an amorphous silicon film obtained by irradiating an amorphous silicon film of 500 Å thickness formed on a glass substrate with a laser beam. FIG. 2 is a photograph taken when observing the surface of the amorphous silicon film with an atomic microscope.
Where a thin-film transistor is fabricated using a thin-film semiconductor, the state of the surface of the thin-film semiconductor is extremely important. This is because carriers are conducted in the surface of the thin-film semiconductor. If corrugations exist in the surface of the thin-film semiconductor, potential barriers, traps, etc. exist which give rise to disconnection or warping of the lattice, the moving carrier being dispersed, trapped, etc.
Also, where a thin-film transistor is fabricated using a thin-film semiconductor, although it is necessary to form a gate insulation film or other insulation film in contact with the thin-film semiconductor, if corrugations exist in the surface of the thin-film semiconductor step coverage of the insulation film is unsatisfactory, causing unfavorable insulation and instability. In addition, the corrugations in the surface of the thin-film semiconductor as described above become hindrances to fabrication of thin-film diodes, photo-electric conversion devices, etc. Consequently, it is preferable that the surface of the thin-film semiconductor be as smooth as possible.